In everyday language depression refers to any downturn in mood, that last for a long period of time,of usually four days or more. This is differentiated from Clinical depression which is marked by symptoms that last two weeks or more and are so severe that they interfere with daily living.

In the field of psychiatry the word depression can also have this meaning but more specifically refers to a mental illness when it has reached a severity and duration to warrant a diagnosis. The Diagnostic and Statistical Manual of Mental Disorders (DSM) states that a depressed mood is often reported as being: "... depressed, sad, hopeless, discouraged, or 'down in the dumps'."

In a clinical setting, a depressed mood can be something a patient reports (a symptom), or something a clinician observes (a sign), or both.

Determinants of mood

Depression can be the result of many factors, individually and acting in concert.

Environment

Reactions to events, often a loss in some form, are perhaps the most obvious causes. This loss may be obvious, such as the loss of a loved one, or having moved from one house to another (mainly with children), or less obvious, such as disillusionment about one's career prospects. A lack of control of one's environment can lead to feelings of helplessness. Domestic disputes and financial difficulties are common causes of a depressed mood. Other causes of depression are climatic conditions, such as a rainy weather and a lack of sunlight, loneliness, and feelings that one isn't cared about by others.

Internal psychological factors

Sometimes the depressed mood may relate more to internal processes or even be triggered by them. Pessimistic views of life or a lack of self-esteem can lead to depression. Illnesses and changes in cognition that occur in psychoses and dementias, to name but two, can lead to depression.

Biological models of causation

Adaptive benefits of depression

While a depressed mood is usually seen as deleterious, it may have adaptive benefits. Of interest is the fact that physical illness tends to lead to depressive behavior and some diseases, such as influenza, are often accompanied by a degree of depression that seems out of proportion to the physical illness. A depressed mood is adaptive in illness in that it leads to the person resting and in general elicits care. Seasonal affective disorder may point to an atavistic link with behavior in hibernation.

Mental disorders with depression

A depressed mood is usually a core feature of some mental disorders such as: